In Nevada, where Paddock lived, anyone who is at least 18 years
old can own a firearm.

Nevada does not license gun
owners, require guns to be registered, limit the number of
guns a person can purchase in one sale, or impose a waiting
period on gun purchases.

Nevada allows the sale of high-capacity magazines, high-caliber
weapons and ammunition, and military-style weapons, though some
other states prohibt the practice.

But one of Nevada's few restrictions is on the sale or
manufacture of automatic weapons, and the Associated Press
reports that Paddock used a legal device to effectively
circumvent that.

The device, known as a "bump stock," replaces the shoulder rest
of a rifle with a device that bounces the weapon back into the
shooter's trigger finger. Effectively, the semiautomatic weapon
still fires one bullet for every pull of the trigger, but the
bump stock automates the trigger-pull process, allowing shooters
to fire at a rate of 400 to 800 rounds a minute.

Gun users can achieve a similar, though less controlled effect by
sticking their finger through their belt loop or simply by
holding a stick between the trigger and the trigger guard.

Instead of a solid stock seen on the gun pictured above, a bump stock bounces back against the shooter's finger, automating the trigger-pulling process.The Slow Mo Guys via Youtube

Paddock rattled off hundreds of shots over few minutes. The
bullets peppered a crowd of 22,000 people, leaving at least 59
dead. More than 500 others were injured either by gunfire or by
trying to escape it.